The present invention relates to a high molecular weight polyolefin part, particularly for an articular prosthesis, which resists friction and creep, as well as to a process for producing said part by shaping, forming or forging in a closed die or mould.
More specifically it relates to parts for a high molecular weight polyolefin articular prosthesis, in which the chains of the polyolefin have a special orientation obtained during the shaping of the parts in the closed mould and which makes it possible to improve the creep and wear resistance of said parts. Articular prostheses, such as total prostheses of the hip are generally constituted by two parts having a hemispherical portion, which rub against one another. These two parts are on the one hand a femoral prosthesis with a joined or unjoined head and tail, the latter being made from a metallic alloy, e.g. stainless steel, cobalt-chromium-molybdenum alloy or a titanium alloy, whilst the head is metallic or ceramic, e.g. of fritted alumina and on the other hand a cotyloid or cup-shaped prosthesis fixed to the pelvis by means of a polymer cement, e.g. of polymethyl methacrylate. The cup-shaped prosthesis can be made from a ceramic material such as fritted alumina, but it is usually made from very high molecular weight polyethylene. Thus, ceramics are fragile materials, which at present limits there development in such prostheses. Therefore, most existing prostheses have a high molecular weight polyethylene cup-shaped prosthesis, their life being estimated as approximately 12 years. The latter does not appear to be limited by the biocompatibility of the material, and instead is due to creep and wear phenomena with respect to the cup-shaped prosthesis. Research is being continued with aim of attempting to increase the life of articular prostheses. To this end, consideration has been given to either changing the materials forming the prosthesis, or at attempting to improve the mechanical properties of existing prostheses. The second solution would appear to be more interesting, because it avoids the need of having to again study the prosthesis, as well as testing the biocompatibility of the materials over a sufficiently long period. Thus, most rejections resulting from poor biocompatibility only appear after being implanted for several years.